1. Field of the Invention
The present invention relates to a power transmitting system for a motor vehicle having a continuously variable transmission and a hydraulic control apparatus for controlling the speed ratio of the transmission.
2. Discussion of the Prior Art
An example of a hydraulic control apparatus for a vehicle continuously variable transmission whose speed ratio is continuously variable, is disclosed in laid-open Publication No. 60-159454. This hydraulic control apparatus includes means for producing a required output pressure corresponding to a currently required acceleration value of a vehicle, and a shift control valve device for controlling the speed ratio of the continuously variable transmission based at least the required output pressure such that the speed of the input shaft of the transmission increases with an increase in the vehicle acceleration value and such that the input shaft speed is higher while a shift lever is placed in a relatively low-gear or engine-braking position, than in a relatively high-gear or normal cruising position.
In the vehicle power transmitting system using the hydraulic control apparatus of the type indicated above, the required output pressure (represented by a suitable parameter such as the amount of opening of the throttle valve of the engine) applied to the shift control valve device is increased when the shift lever is operated to the relatively low-gear position. As a result, the speed ratio of the transmission (ratio of the input shaft speed to the output shaft speed) is increased toward the maximum value by the shift control valve device, so as to shift-down the transmission, with the input shaft speed being raised. During a low-speed running with the shift lever placed in the low-gear position, therefore, the engine speed is raised, whereby a relatively high engine braking effect may be obtained. When the speed ratio of the transmission is higher than a given value and the vehicle running speed is relatively low, the speed ratio is controlled so as to lower the engine speed with an increase in the speed ratio, whereby a suitable engine braking effect may be provided.
The shift control valve device of the above hydraulic control apparatus includes a valve spool adapted to receive a thrust corresponding to the throttle valve opening so that the spool is biased by this thrust in the direction to shift down the transmission. The spool also receives an input shaft speed pressure corresponding to the input shaft speed of the transmission which is detected by a pitot tube. The spool is biased by this input shaft speed pressure in the direction to shift up the transmission. Accordingly, the spool is positioned under equilibrium of forces between the thrusts which act on the spool in the opposite directions (shift-down and shift-up directions}, so that the speed ratio of the transmission is changed as the spool is moved. During reverse running of the vehicle, however, the pulleys of the transmission are rotated in the direction opposite to that during forward running of the vehicle, and the pitot tube almost fails to produce the input shaft speed pressure. As a result, the speed ratio of the transmission is changed in the shift-down direction toward the maximum value by the shift control valve device, whereby an excessively large engine braking effect is provided during reverse running of the vehicle at a relatively low speed. This deteriorates the driving comfort.